Typically, marine speedometer systems use either pitot tube or paddle wheel type devices to measure vessel speed. The pitot tube senses stagnation pressure as a function of speed, while the paddle wheel supplies output pulses that are counted to indicate speed. The devices are usually mounted on the stern or the bottom of the vessel. It is common in the arm to provide redundant marine speedometer systems as a backup for accuracy or dependability. Unfortunately, there are times when both the primary and redundant marine speedometer systems are inoperative. Problems occur most frequently when using a pitot tube on weedy inland lakes. When the pitot tube is partially clogged or blocked by weeds or other debris, a lower than actual speed will be indicated by the marine speedometer system. The paddle wheel devices experience problems in the form of mechanical wear, corrosion and inaccuracy due to rotational resistance.